c. 1300, "living in the world, not belonging to a religious order," also "belonging to the state," from Old French seculer (Modern French séculier), from Late Latin saecularis "worldly, secular, pertaining to a generation or age," from Latin saecularis "of an age, occurring once in an age," from saeculum "age, span of time, lifetime, generation, breed."
This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to words for "seed," from PIE root *se- "to sow" (see sow (v.), and compare Gothic mana-seþs "mankind, world," literally "seed of men").
Used in ecclesiastical writing like Greek aiōn "of this world" (see cosmos). It is source of French siècle. Ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years). In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
1846, "one who theoretically rejects and ignores all forms of religion based on revelation;" see secularism + -ist. From 1851 as "one who maintains that public education and civil policy should be conducted without the introduction of a religious element." Related: Secularistic.
Others are reading
Definitions of secularism from WordNet
a doctrine that rejects religion and religious considerations;